California is one of the most geographically and ecologically
diverse regions in the world.
Despite its variety of landscapes, much of California
experiences a Mediterranean climate
similar to other low and midlatitude regions
on the west sides of continents, including the Mediterranean
Basin and parts of South Africa, Australia, and Chile.
All of these regions share cool winters
with intermittent wetness and hot, dry summers.
This type of climatic regime has characterized California
for millions of years.
The distinctness of this climate pattern from all nearby areas
in each case makes each of these Mediterranean climate regions
a bit like an island.
It's allowed the evolution and assembly
of many endemic species and unique ecological communities.
What does a Mediterranean climate look
like in California?
It depends somewhat on where in the state you look.
In these figures, the months of the year are along the bottom
and the columns are for five cities, Sacramento
in the northern Central Valley, San Francisco on the north
Central Coast, Bakersfield in the southern Central Valley,
San Diego on the South Coast, and El Centro
in the interior southern desert near California's borders
with Mexico and Arizona.
The shading and the precipitation graphs
shows a typical Mediterranean climate in each location,
with summer droughts in the middle of the graph,
typically from about May to September,
and winter rains out at the tails of each graph.
This basic pattern is consistent across nearly all of the state,
with some exceptions far to the east,
where summer rains are more frequent and important
in the desert.
If you think for a moment about life as a plant in California,
this basic pattern presents a challenge.
The warmest months of the year with the longest daylight
are also the driest.
Because of this, many California ecosystems
have peak productivity in the early to midspring,
rather than the summer.
The comparison among cities shows
you some important patterns.
First, winter rainfall is higher in the north than in the south
and higher at the coast than in the interior.
Second, this temperature graph shows
that it's generally warmer and summer and cooler
in the winter.
An important exception to this though
can be when summer fog lowers temperatures
right at the coast.
This relates to a third overall pattern,
which is that the coast has the least extreme temperature
swings, while interior cities, like Sacramento and El Centro,
are both hotter in summer and cooler in winter.
The basic annual pattern of temperature and precipitation
throughout the year is the same in nearly all parts
of the state.
But the variations on this theme across the state help
explain the diversity of ecosystems
that exist in California.
Patterns of precipitation and temperature
are closely tied to latitude, topography,
and atmospheric conditions.
The broad scale view of surface air temperature in California
shows warming as you go from north to south.
And precipitation generally declines
from north to south as well.
But there's a lot of complexity and spatial variation
in both temperature and precipitation that can't be
explained simply by latitude.
This arises partly from the inherent structure
within the atmosphere and partly because
of the influence of California's varied setting and complex
landscape.
In particular, you can see that both winter and summer
temperatures and precipitation track elevation closely.
And although it's hard to see at this scale,
the ocean-facing western slopes of mountains in California
tend to be wetter than the leeward east-facing slopes.
When moisture-laden air within storm systems
sweeps over mountains, air is forced upward
in a process known as orographic uplift,
forming clouds and precipitation along the windward side
of the mountain range.
Orographic rainfall is greater on the windward side
of mountain ranges and produces a dryer rain shadow effect
on the leeward side.
This is because winter air masses
laden with moisture move from the Pacific
towards the interior with the prevailing winds blowing
through the northwest in California.
As they're blown east, they encounter mountain ranges
that force them up and over.
Along the way, these air masses cool adiabatically
because of the drop in pressure that they experience
with increasing altitude.
Cooling reduces their dew point, wringing precipitation
out of them as they rise up the western slopes of California's
mountains.
East of the mountaintops, these air masses sink down,
heating as their pressure increases,
and holding on to the remaining water vapor.
The rain that is wrung out of air masses
rising up the windward sides of mountains
is called orographic rainfall.
And the phenomenon of reduced rainfall
on the leeward sides of mountains
is referred to as a rain shadow.
In California, air masses repeatedly
climb and descend mountains as they move east from the ocean,
producing a general pattern of higher rainfall near the coast
than in the interior.
In this cross-section of Central California,
you can see the decline in precipitation
from the coast to Hollister, east of the first coastal
range, then the decline again to Coalinga
in the shadow of the interior coast range.
Fresno picks up a little more moisture
as air masses began to rise up the Sierra foothills.
And Grant Grove experiences high levels of orographic rainfall
at mid-elevations in the Sierra.
At high elevations in the Sierra Nevada,
natural water storage in the form of snow pack
allows gradual runoff to occur and has important implications
for many ecosystems.
Finally, Independence lies in the rain shadow of the Sierras.
And Death Valley, behind two more mountain ranges
and at the lowest elevations in the Western Hemisphere,
can sometimes go more than a year
without any precipitation reaching it.
Although we talk about annual averages,
precipitation varies strongly from year
to year in California.
Monthly precipitation commonly ranges from less than 50%
to over 150% of its long-term average.
So there's really no such thing as an average rainfall
year in California.
Reconstruction of the past 400 years of California winter
precipitation from Blue Oak tree ring analysis
indicates that the high level of interannual variability
observed in recent decades is not unusual.
This reconstruction shows annual estimates in blue
and a five-year smooth version is plotted in black.
Year to year variation in California's precipitation
is driven by several factors, including
interannual and decadal cycles like the Pacific Decadal
Oscillation and the El Nino Southern Oscillation.
The North Pacific high pressure system,
which normally sits between the West
Coast of North America and the Hawaiian Islands,
has the most immediate impact on California's climate.
A network of subtropical high pressure centers,
including the North Pacific High,
result from the uneven heating of the Earth's surface
and cause high pressure systems near midlatitudes.
Instead of forming a continuous belt of high pressure,
land masses cause disruptions that
result in high pressure centers such as the North Pacific High.
These satellite images show the position of the North Pacific
High during summer and winter.
And the yellow lines are isobars that show surface pressure.
The North Pacific storm track becomes less active and recedes
northward during summer months, while the North Pacific
High is most intense and fends off incoming storms.
Furthermore, descending air within the High,
which warms adiabatically and increases in dew point,
inhibits rainfall, creating dry summer conditions
throughout California.
During winter months, when the North Pacific High
weakens and shifts to the southeast,
the North Pacific storm track migrates
southward and intensifies.
As a consequence, winter storms are more intense
and more likely to directly impact California
because the North Pacific High is not able to block them
the way it does in summer.
A key feature that distinguishes California
from other regions of the United States
is a low-level temperature inversion.
Normally, air temperature cools with increasing altitude.
But below the ceiling of a temperature inversion,
air temperature actually increases as you go higher.
The density of air under the inversion
is vertically stacked and very stable.
So the low-level temperature inversion
forms a cap that inhibits vertical mixing of surface air.
This graph shows the vertical temperature
at Oakland, California during July,
with a strong North Pacific High.
Above about 0.6 kilometers, air temperature
declines with altitude.
Below the inversion ceiling, at about 0.6 kilometers
or 600 meters altitude, the opposite pattern occurs.
It's colder near the ground, except for the very bottom
0.1 to 0.2 kilometers within the layer of buildings.
The relatively dense air, up around 1 kilometer,
traps the warm air below it, stabilizing the inversion
ceiling.
This both contributes to the development
of an atmospheric marine layer at the coast
and traps particulates and pollution.
Controlled in part by the North Pacific High,
these temperature inversions are a persistent feature
along the California coast and inland valleys.
At the coast, a coastal subsidence inversion
is caused by descending air in the North Pacific High,
combined with the cool air immediately
above the relatively cold coastal ocean.
During strong inversions, the afternoon temperature
at San Diego near sea level along the coast
can be 8 degrees Celsius cooler than the adjacent mountains,
at about 500 meters elevation, just inland.
The inversion stability also causes extensive marine stratus
clouds or fog, which reflect solar radiation
and moderate temperature and humidity.
In addition, the higher heat capacity
of the ocean compared to land means
that the ocean maintains an air-conditioned coastal land
zone that pervades across the coastal lowlands to locations
inland where marine air can easily penetrate, moderating
both high and low temperatures.
Inland valleys experience inversion
more often in the winter months due to lower levels
of solar energy reaching the Earth's surface, cooling
of land surfaces, and drainage of cool air
into low-lying areas.
First, fog forms over the ocean.
Heating inland causes the air to rise, pulling in below it
the moisture-laden marine air.
Fog get sucked through the valley at the Golden Gate
into the San Francisco Bay in Central Valley
and also through valleys such as the Salinas
Valley across the coast range, filling them also with fog.
Fog largely blows with the wind from the northwest.
So northern Monterrey Bay can sometimes
be fog-free because the fog flows southeast and misses
the bend at the bay.
In spring and summer, winds in the coastal region
usually blow from the northwest.
These prevailing northwesterly winds
can be disrupted along the Southern California
Bight, the section of coastline from Point Conception
to San Diego, when winds turn south during periods
when the Catalina Eddy takes hold.
This eddy forms in the lee of the mountainous coastline,
which takes an abrupt change in direction at Point Conception.
This is another way topographic complexity,
via its effects on atmospheric dynamics,
shapes the spatial diversity of climate in California.
Next to the annual cycle, the most important pattern
of climate variation in California
is the El Nino Southern Oscillation or NSO.
El Nino and its opposing phase partner, La Nina,
are parts of a coupled ocean atmosphere phenomenon
rooted in the tropical Pacific and involving fluctuations
in heat content, ocean surface temperature, and trade winds.
The trade winds or surface winds near the equator
generally blow from east to west on the Pacific,
transporting ocean surface water away
from the coast of South America and driving upwelling.
During normal years, sea surface temperature increases
from east to west along the equatorial Pacific,
with relatively cold water along the South American coast
and very warm water in the western tropical Pacific.
About every two to seven years, the trade winds slacken
or reverse direction, increasing the sea surface temperature
in the central and eastern tropical Pacific.
In response, there's less upwelling along the South
American coast.
The resulting changes in regional heat
and moisture impact global atmospheric circulation
patterns.
One primary impact is a shift and intensification
in part of the jet stream across the northern Pacific
that bring winter storms further south during winter,
toward California.
As a result, California can experience, but doesn't always,
more frequent and intensive winter storms and precipitation
during an El Nino event.
In contrast, La Nina events occur
when trade winds become stronger,
limiting precipitation in California
and generally making it dryer.
Both La Nina and El Nino have the strongest effects
in Southern California and fade in their effects
as you go north into Northern California.
Let's take a step back from the complexities
of California's climate variability
to explore relationships among climate, atmospheric dynamics,
and air pollution in California.
Remember that the temperature inversion
can trap fog and low clouds, as well
as particulates and pollutants.
California's persistent temperature inversion
can increase the effects in the state of air pollution
on both health and ecological systems.
Here we can see smog, trapped by temperature inversions
and surrounding mountains in the Los Angeles Basin and the San
Joaquin Valley.
Since the mid-1940s, air quality in California
has been among the worst in the nation.
The infamous Los Angeles smog is a result
of uncontrolled emissions from sources
like industry and automobiles.
Photochemical smog is produced when these pollutants react
under high temperatures, light intensity,
and thermal inversions, forming thousands of secondary chemical
compounds, including highly toxic ozone.
Air quality has improved since the 1970s because of strict air
pollution measures and accompanying technological
advances.
In this graph, ROG are reactive organic gases and NOx
are nitrous oxides, two major components of air pollution.
However, emissions of particulate matter
have not decreased.
PM10 and PM25 are two sizes of particulate matter
that remain steady in California over the last 40 years.
Ozone is another important pollutant in California
and other urbanized regions.
Although ozone levels have improved in the state,
ozone still causes serious ecological and human health
effects in California.
The maps show that ozone concentrations have decreased
over the past 20 years, from the map on the left
to the map on the right.
Areas in yellow to red show ozone levels that exceed
federal air quality standards.
Although they are lower today, with reduced extent
of exceedance levels and lower peaks,
California still has the highest ozone air pollution
and associated highest risks to human health in the US.
Ozone damage to plants also results
in decreased photosynthesis.
And interactions with various other stressors
influence the extent of these ozone phytotoxic effects.
As these maps show, ozone can be transported long distances,
affecting areas far from cities and industry.
California's ecosystems are naturally nitrogen-limited.
Yet chronic nitrogen deposition from industry, vehicle exhaust,
and fertilizer production and use is a problem.
It leads to excess nitrogen, causing acidification
and eutrophication, with effects like harmful blooms
in aquatic systems and plant species
losses in terrestrial systems.
Species adapted to low nutrient or oligotrophic environments
are especially sensitive to atmospheric nitrogen.
In low productivity ecosystems, such as grasslands,
coastal sage scrub, and desert scrub,
nitrogen deposition can enhance the growth of invasive plant
species, increasing their dominance
and reducing native plant species
abundance and diversity.
Movement and deposition of nitrogen compounds
is much more spatially restrictive than ozone
distribution because of nitrogen's high deposition
velocity.
Most nitrogenous compounds fall out onto the land surface
relatively quickly and near their sources.
Consequently, steep landscape gradients
of nitrogen deposition occur in California mountain ranges,
with the highest potential for negative effects near emission
source areas like the Los Angeles region and the Central
Valley.
Some of these levels of background deposition,
despite policies to curve nitrogen pollution,
remain as high as or higher than the amounts of nitrogen
that farmers put on conventional crop fields every year.
Ozone and nitrogen deposition, which
includes NOx or nitrous oxides, are the most ecologically
damaging pollutants, while ozone and particulates
pose the greatest threats to human health.
But perhaps the greatest threat that these substances
pose to California's ecosystem is
due to their direct role in driving climate change.
This photo shows the effect of the 2013
to 2015 drought on Lake Oroville, a reservoir
in the South Fork of the Feather River near Chico, California.
It underscores California's reliance on the climate
through features like water supply.
Since the beginning of the 20th century,
California has warmed by approximately 0.9 degrees
Celsius, which is close to the global average level
of warming.
Annual average precipitation over California
has not changed consistently.
But the warmer temperatures tend to increase
the fraction of precipitation falling as rain rather
than snow and also to increase evapotranspiration losses
of water from ecosystems, resulting in net drier
ecosystems and lower summer moisture availability
regardless of precipitation amounts changing.
As climate warms, many species are expanding upward
in elevation or north in latitude.
One such example comes from the Deep Canyon Transect,
an elevational gradient in the Southern California desert
that was surveyed in 1977 and again in 2007.
10 dominant plant species characterize the transect
as it rises 2,000 meters from desert scrub to conifer forest.
While no shifts occurred in range limits,
a symmetrical shift to greater upslope cover
occurred in dominant plant species.
Similarly, latitudinal shifts are occurring in response
to warming climates.
For example, invertebrate communities
of the rocky intertidal system in Monterrey Bay
contain more southern species and fewer northern species
than they did several decades ago.
California's topographic relief increases the probability
that some species will be able to track
climate change While many species will
be able to capitalize on this topographic opportunity,
some others probably will not.
Some species cannot move fast enough, or are too long lived,
or will encounter soil, or competitive,
or other constraints that prevent them from keeping up
with shifting ranges.
High elevation species could face
a decrease in the area of suitable high elevation
habitats available to them.
And some species could literally be pushed off
the top of mountains.
The American pica, shown here, is a subject
of much debate over whether it will adapt behaviorally
and evolutionarily to climate change
or disappear off the tops of California's highest mountains.
In addition to considering rates of movement
for single organisms, we also have
to think about biomes or major types of ecosystems moving.
Biomes have managed to keep up with historical velocities
of climate change.
But anthropogenic warming is 10 to a hundred times more rapid.
The map on the left shows vegetation classes
over California for a 1961 to 1990 baseline period.
And the map on the right shows predictions
for 100 years later.
Noticeable expansions are predicted for mixed evergreen
forests and grasslands.
Movements today are constrained by the new challenges
of extensive human development.
And as organisms and biotypes shift,
opportunities for species invasions and disturbances
like fire could change dramatically as well.
In most parts of the world, but especially in California,
climate change impacts on ecosystems
occur in the context of a wide range of interacting
anthropogenic impacts.
The impacts on California of climate changes to date
are already widespread and consequential.
And those in a plus 2 degrees Celsius
world will be much larger.
This graph shows observed and predicted temperatures
relative to 1986 to 2005.
Projections are for two scenarios.
The blue line is a scenario with aggressive mitigation
of greenhouse gas emissions.
And the red line is a business-as-usual scenario.
But a future with continuing business-as-usual emissions
and global average temperatures in 2100
on the order of 4 degrees Celsius
over preindustrial levels would be so different
that the tools for projecting and describing the conditions
become inadequate.
The lack of information about the future,
especially about the plus 4 degrees Celsius world,
is heavily shaped by our almost total inexperience
and inability to characterize the effects of such exceedingly
fast changes in the context of a wide range of interacting
ecological and anthropogenic effects.
In summary, here are the main points that we've
covered on the diversity of California's climates,
atmospheric conditions, and pollutants
and on the state's changing climate.
First, California's cool, wet winters and hot, dry summers
present unique challenges to organisms limited
by water and temperature.
Seasonal and interannual precipitation patterns
in California are strongly linked
to seasonal variations in the North Pacific High
and in extratropical storm tracks.
Third, annual precipitation amounts throughout California
vary significantly over a range of time scales.
Some of this variability stems from the occurrence
of El Nino Southern Oscillation and La Nina events.
Spatial variation across California
in temperature and precipitation is strongly
shaped by latitude from north to south and topography
throughout the state, but especially from east to west.
This spatial variation is part of what
gives rise to California's ecosystem diversity.
Pollutants and particles, most notably
ozone, nitrogen compounds, and dust,
are serious threats to ecosystems and biota,
as well as humans.
Temperature inversions prevent vertical mixing and trap
pollutants and particles.
Species responses to climate change across the state
include elevation and latitudinal shifts
in terrestrial and marine environments,
as well as other changes described in the readings.
The impacts on California of climate changes to date
are ready widespread and consequential.
Those in a plus 2 degree Celsius world,
which would require aggressive mitigation,
will be much larger.
Those in a plus 4 degree Celsius world,
the business-as-usual future, are really
difficult to even characterize.
